Anti-M/NEI antibodies enable researchers to detect and measure the M/NEI antigen in biological samples. This target is a reported synonym of the SERPINB1 gene, which encodes serpin family B member 1. The human version of M/NEI has a canonical amino acid length of 379 residues and a protein mass of 42.7 kilodaltons, although 2 isoforms have been identified. It is reported to be localized in the cytoplasm and is secreted of cells and notably expressed in the tonsil, spleen, rectum, and bone marrow. Other names for this target antigen include EI, ELANH2, and HEL-S-27.